Transient tension changes initiated by laser temperature jumps in rabbit psoas muscle fibres.

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Abstract
1. A technique was developed to generate 2‐8 degrees C step temperature perturbations (T‐jumps) in single muscle fibres to study the thermodynamics of muscle contraction. A solid‐state pulsed holmium laser emitting at 2.065 microns heated the fibre and surrounding solution in approximately 150 mus. The signal from a 100 microns thermocouple fed back to a heating wire maintained the elevated temperature after the laser pulse. 2. Tension of glycerol‐extracted muscle fibres from rabbit psoas muscle did not change significantly following T‐jumps when the fibre was relaxed. 3. In rigor, tension decreased abruptly on heating indicating normal (not rubber‐like) thermoelasticity. The thermoelastic coefficient (negative ratio of relative length change to relative temperature change) of the fibre was estimated to be ‐0.021 at sarcomere lengths of 2.5‐2.8 microns. Rigor tension was constant after the temperature step and returned to the original value on recooling. 4. In maximal Ca2+ activation, tension transients initiated by T‐jumps had several phases. An immediate tension decrease suggests that thermoelasticity during contraction is similar to that in rigor. Active tension then recovered to the value before the T‐jump with an apparent rate constant of approximately 400 s‐1 (at 10‐20 degrees C). This rate constant did not have an appreciable dependence on the final temperature. Finally, tension increased exponentially to a new higher level with a rate constant of approximately 20 s‐1 at 20 degrees C. This rate constant increased with temperature with a Q10 of 1.4. 5. At submaximal Ca2+ activation the tension rise was followed by a decay to below the value before the T‐jump. This decline was expected from the temperature dependence of steady pCa‐tension curves. The final tension decline occurred on the 1‐5 s time scale. 6. The value and amplitude dependence of the rate constant for the quick recovery following T‐jumps were similar to those of the quick recovery following length steps during active contractions. The enthalpy change associated with the quick tension recovery following temperature‐step perturbations was estimated to be positive suggesting that the recovery process is an endothermic reaction. Slower reaction steps on the 10‐30 ms timescale, as well as reactions corresponding to the quick recovery, may contribute to the cross‐bridge power stroke.